This invention relates to improved hydrocarbon recovery from a Fischer-Tropsch process.
Several natural gas vehicles (NGVs) have been developed in response to recent laws such as the Clean Air Act (1990) and the Energy Policy Act (1992). Because natural gas tends to burn cleanly, NGVs are a potential alternative to gasoline-powered vehicles.
There are several commercially available natural gas-based fuel formulations. The main formulations are: compressed natural gas (CNG), liquefied natural gas (LNG), and liquefied petroleum gas (LPG). In CNG technology, the gaseous (natural gas) fuel is stored at very high pressures of about 20684 kPa to 24132 kPa (3000-3500 psia). This technology is limited because the vehicles tend to have a relatively short driving range (due to low energy storage per fuel storage container volume), the high storage pressures pose a safety concern, the fuel storage containers tend to be relatively heavy; and the refueling stations tend to be relatively expensive.
LNG technology overcomes some of these limitations, in particular, by providing much more energy per unit volume, lower vehicle fuel system weight and higher fuel storage volume capability. However, the costs of the fuel storage containers are still relatively high, and the need to deliver pressurized natural gas to an engine""s fuel injectors adds to the complexity and cost of the fuel delivery system.
Liquefied petroleum gas (LPG), which includes mostly propane, n-butane and/or i-butane, with small amounts of pentane, is an alternative to LNG and CNG which provides similar clean burning characteristics and which overcomes the limitations of both CNG and LNG. LPG provides higher energy storage per vessel volume than either CNG or LNG and operates at relatively low pressures (about 827 kPa (120 psia)), as compared to CNG, and at ambient temperatures. A limitation of using LPG is that the LPG supply is limited and LPG is much more expensive than LNG.
It would be advantageous to provide new methods for preparing LPG. The present invention provides such methods.
An integrated process for improved hydrocarbon recovery is disclosed. The process involves isolating a methane-rich stream for use in hydrocarbon synthesis. The stream is derived from a well gas from a natural gas source. The well gas may also include ethane, an LPG stream (including mainly C3-C5 hydrocarbons) and optionally a C5+ stream (xe2x80x9cwell gas condensatexe2x80x9d). The methane-rich stream is isolated in a first separation zone, along with an LPG stream. Either prior to or following separation in the first separation zone, one or more of the streams may be treated in a hydroconversion reaction zone to remove sulfur compounds contained therein.
The methane-rich stream is converted to syngas and subjected to a hydrocarbon synthesis step, for example, a Fischer-Tropsch synthesis step. The products from the hydrocarbon synthesis step typically include a C1-C4 fraction, at least one low-boiling liquid fraction (generally in the C5-C20 range), and a high-boiling wax fraction (generally in the C20+ ). These fractions are isolated in a second separation zone. The C4xe2x88x92 fraction from the synthesis step is combined with the well gas for recovery of LPG and a methane-rich stream (for production of syngas).
The low-boiling liquid fraction and the high-boiling wax fraction are preferably subjected (either separately or in combination) to additional process steps, for example hydrotreatment, hydroisomerization, hydrocracking (particularly in the case of the wax fraction), and the like (hereafter referred to as hydroconversion). The products of the additional process steps are sent to a third separation zone, and yield compositions useful, for example, in fuel-related products (preferably C5-C20 hydrocarbons) and an additional C4xe2x88x92 fraction. The additional C1-C4 fraction can also be sent to the first separation zone and treated in an analogous fashion to the C1-C4 fraction from the hydrocarbon synthesis.
In one embodiment, a mixed LPG fraction is recovered from the first separation zone. In a separate embodiment, individual ethane, propane, and butane streams are isolated rather than the mixed LPG fraction. In this embodiment, the first separation zone preferably uses a demethanizer to isolate methane, a de-ethanizer to isolate ethane, and a de-propanizer to isolate propane and butanes.